In this observational study of Black women, vigorous physical activity in adulthood was inversely associated with depressive symptoms. The biggest decrease in the odds of depressive symptoms was observed among women who were active in both high school and adulthood. Women who were active in high school only or adulthood only still had significantly lower odds of depressive symptoms relative to women who were never active.
Walking for exercise was not associated with risk of depressive symptoms overall, but there was some evidence of an in-verse association, albeit weak, among obese women (BMI 30+). Although obese women were less likely to be active in general, those who were active were more likely to engage in walking as opposed to vigorous physical activity. If causal, the association of walking with fewer depressive symptoms in obese women may have important public health implications, because few obese individuals appear able to perform or sustain high levels of vigorous physical activity. Moreover, physiologically, based on perceived and measured exertion, brisk walking among obese individuals might be equivalent to “vigorous” physical activity (44
Our findings agree with 11 prospective observational studies that have reported a protective effect of physical activity on depressive symptoms (9
). Five other studies found no association (21
). To our knowledge, none of these studies stratified by BMI to assess whether certain forms of physical activity might be more beneficial in obese compared with nonobese women. One study reported an inverse association between walking and depressive symptoms among people ages 65 and older (14
); small numbers of women (n
= 455) limited our ability to examine the influence of walking in that age group. Our finding of an inverse association of depressive symptoms with vigorous physical activity in high school is compatible with three studies of young athletes (12
). Our findings also agree with most secondary prevention clinical trials of exercise in the treatment of depression (46
The BWHS is the first large study of U.S. Black women to assess the relation between physical activity and depressive symptoms. We used the average of two physical activity variables measured at two separate points in time (1995 and 1997), which may provide a more valid measure of long-term physical activity than a measure taken at a single point in time (36
). Data on physical activity during high school allowed us to examine changes in exercise patterns over time.
Several limitations to this study are worth considering. First, although our study was prospective in design, it was not possible to determine whether physical activity levels were antecedent to, or consequences of, depression. Our CES-D measure was self-administered in 1999 only. Because it was not administered in 1995, we could not use the CES-D scale to identify and exclude women reporting depressive symptoms at baseline. However, by excluding women who reported physician-diagnosed depression before 1999, we attempted to minimize the possibility that depression influenced levels of physical activity.
Second, as with any observational study in which the exposure is not randomly allocated, the association between physical activity and depressive symptoms may have been confounded by other unmeasured factors. We were able to measure and control for several potential confounding variables (e.g., preexisting health conditions, smoking) and results were similar whether or not we included these variables in the model.
Third, our study focused on leisure time physical activity and did not include other forms of physical activity that may be related to depression, such as occupational and utilitarian activity. In addition, we did not have extensive detail on the actual type of physical activity (e.g., basketball vs. aerobics) or its intensity (e.g., brisk vs. slow walking), which may have led to some misclassification of activity levels. The lack of a monotonic dose-response relation may suggest a threshold effect for vigorous physical activity or may indicate that women reporting levels of vigorous physical activity in excess of 5 hr per week represent a distinct group of individuals with a greater risk of depression (e.g., women with low self-image). The curvilinear pattern may also be the result of reporting error if, for example, women with underlying depression were more likely to over-report their activity levels relative to nondepressed women.
Fourth, although the CES-D scale has been widely used in community-based samples to screen for depression, it is not a definitive clinical measure of depression. In one study, a cutoff of “16 or more” identified 65% of women with clinical depression (40
). Despite its inability to capture all women with depression, the CES-D scale is practical for large epidemiologic studies in which the administration of a more structured clinical interview is not feasible. Furthermore, results were similar when we used a cutoff of 25 or greater, which was more likely to identify women with clinical depression.
Finally, it is possible that women who developed depression after baseline were more likely to withdraw from the study. When we compared those included in this analysis to those not included with respect to baseline physical activity measures, in addition to demographic, social, and lifestyle characteristics, we did not observe any important differences. Prevalence estimates for vigorous physical activity and walking for exercise in the BWHS are consistent with estimates from nationally representative studies of Black women (26
). The proportion of women who reported depressive symptoms (CES-D 16+) in our sample (27%) is higher than that found in the general population normative sample (21%) (30
), but is compatible with estimates found for Black women in other studies (4
Although study participants reside in more than 14 U.S. states, the BWHS is a convenience sample and was not designed to be representative of the general population of U.S. Black women. In particular, the study cohort underrepresents women with less than a high school education. However, the observed associations persisted within strata of education—as well as age and geographic region—suggesting that they might extend to the general population of U.S. Black women ages 21 to 69 years.
Several mechanisms could explain a protective effect of physical activity on depressive symptoms. Physical exercise influences the central dopaminergic, noradrenergic, and serotonergic systems (28
). The increased biosynthesis of neurotransmitters, including monoamines (28
), catecholamines (49
), and endorphins (50
), may improve mood. Social–psychological mechanisms include enhanced self-efficacy and self-esteem, improved self-image and self-worth, and decreased social isolation (10
). Physical activity may also boost immune function (52
), and prevent chronic conditions (e.g., hypertension and diabetes) associated with depression.
In summary, our results suggest that vigorous physical activity reduces depressive symptoms in U.S. Black women, and that despite a history of inactivity, the initiation of a current physical activity program could have mental health benefits. In the context of primary prevention research, additional studies are needed to clarify the optimal frequency and intensity of exercise needed to have an effect. According to data from the NHANES study, the prevalence of leisure time activity among Black women in the United States is about half that of White women (26
). Interventions aimed at increasing physical activity among Black women will need to target groups with lower exercise levels, such as women with low educational attainment and obese women.